Unexplained Flooding

We've been having problems with flooding of a new extension caused by the storms during the summer. We have a combined sewer system and we are the last house at the bottom of a sloping close. During heavy rain the sewer can't cope and backs up and overflows the road drains. When this happens we have water coming in between the floor and walls of the new extension. The builder has been and checked everything is sealed and as it should be. Neighbours have mentioned there being a soakaway in our back garden that takes some or all of the streets rainwater. For this to happen there would have to be a pipe running under the original house. My thought is that maybe this pipe is broken and is leaking water into the void under the new floor and when it rains hard enough it comes into the kitchen.
Would this be possible or is there another possible explanation?
The original house was built in the 50s.

Thanks in advance.